The present invention relates to underfll compounds including electrically charged filler elements, microelectronic devices having underfill compounds including electrically charged filler elements, and methods of disposing underfill compounds having electrically charged filler elements on microelectronic devices.
Microelectronic device assemblies, such as memory devices and microprocessors, typically include one or more microelectronic components attached to a substrate, The microelectronic components commonly, include at least one die having functional features such as memory cells, integrated circuits, and interconnecting circuitry. The dies of the microelectronic components may be encased in a plastic, ceramic, or metal protective covering. Each die commonly includes an array of very small bond-pads electrically coupled to the functional features. The bond-pads can be used to operatively connect the microelectronic component to the substrate.
One type of microelectronic component is a xe2x80x9cflip-chipxe2x80x9d semiconductor device. These components are referred to as xe2x80x9cflip-chipsxe2x80x9d because they are typically manufactured on a wafer and have an active side with bond-pads that initially face upward. After manufacture is completed and a die is singulated, the die is inverted or xe2x80x9cflippedxe2x80x9d such that the active side bearing the bond-pads faces downward for attachment to a substrate. The bond-pads are usually coupled to terminals, such as conductive xe2x80x9cbumps,xe2x80x9d that electrically and mechanically connect the die to the substrate. The bumps on the flip-chip can be formed from solders, conductive polymers, or other materials. In applications using solder bumps, the solder bumps are reflowed to form a solder joint between the flip-chip component and the substrate. This leaves a small gap between the flip-chip and the substrate. To enhance the integrity of the joint between the microelectronic component and the substrate, an underfill material is introduced into the gap. The underfill material bears some of the stress placed on the components and protects the components from moisture, chemicals and other contaminants. The underfill material can include filler particles to increase the rigidity of the material and modify the coefficient of thermal expansion of the material.
The underfill material typically is dispensed into the underfill gap by depositing a bead of the underfill material along one or two sides of the flip-chip when the underfill material is in a fluidic state (i.e., flowable). As shown schematically in FIG. 1, a bead of an underfill material U may be dispensed along one side of the die D. The flowable underfill material will then be drawn into the gap between the die D and the substrate S by capillary action. The direction of this movement is indicated by the arrows in FIG. 1. After the underfill material fills the gap, it is cured to a hardened state. Although such a xe2x80x9csingle strokexe2x80x9d process yields good results, the processing time necessary to permit the underfill material U to flow across the entire width of the die can reduce the throughput of the manufacturing process.
FIG. 2 illustrates an alternative approach wherein the underfill material U is applied in an L-shaped bead along two adjacent sides of the die D. By reducing the average distance that the underfill material has to flow to fill the underfill gap, processing times can be reduced. The L-stroke approach, however, can lead to more voids in the underfill material, which adversely affect the integrity of the bond between the die D and the substrate S.
In the single stroke and L-stroke approaches, the filler particles can become segregated from the polymer fluid as the underfill material flows across the die. Consequently, one side of the flip-chip often has a greater concentration of filler particles. The nonuniform distribution of filler particles creates differences in the rigidity and the coefficient of thermal expansion of the underfill material across the die.
In other embodiments, the underfill material may be deposited across a plurality of dies at the wafer-level to form an underfill layer. After the underfill layer is formed, the dies can be singulated and attached to substrates. Forming an underfill layer with filler particles on a die before attaching a substrate to the die has some drawbacks. For example, the filler particles in the portion of the underfill layer above the conductive bumps can obstruct the connection between the conductive bumps of the die and the substrate. To prevent the filler particles from interfering with the connection, one approach is to form two underfill layers on the die. The first underfill layer includes filler particles and has a thickness no greater than the height of the conductive bumps. The second underfill layer is formed over the first layer and does not contain filler particles. This approach, however, requires two dispensers and two types of underfill material. Another approach is to form the underfill layer on the die at the wafer-level before forming the conductive bumps. Next, vias are formed in the underfill layer and the conductive bumps are formed in the vias. This approach, however, is complicated and can result in contamination of the underfill layer and/or the conductive bumps. Moreover, it is difficult to deposit solder paste in very small vias. Another approach is to form the underfill layer over the die and the conductive bumps, then remove the top portion of the underfill layer so that the underfill layer has a thickness equal to the height of the conductive bumps. This approach also is complicated, requires cleaning, and may contaminate the device. Accordingly, a new method for forming an underfill layer that has filler particles is needed.
The present invention is directed to underfill compounds including electrically charged filler elements, microelectronic devices having underfill compounds including electrically charged filler elements, and methods of disposing underfill compounds including electrically charged filler elements on microelectronic devices. One aspect of the invention is directed to a composition for use in an underfill layer of a microelectronic device. In one embodiment, the composition includes a flowable binder and a plurality of electrically charged filler elements disposed within the flowable binder. The electrically charged filler elements can include silica, silicon nitride, aluminum oxide, and/or aluminum nitride. The flowable binder can include a liquid polymer.
Another aspect of the invention is directed to a microelectronic device. In one embodiment, the microelectronic device includes a microelectronic component, a plurality of electrical couplers carried by the microelectronic component, and an underfill layer covering at least a portion of the plurality of electrical couplers. The underfill layer comprises a binder and a plurality of electrically charged filler elements in the binder. In one aspect of this embodiment, the underfill layer also includes a first zone having a first concentration of electrically charged filler elements and a second zone having a second concentration of electrically charged filler elements different from the first concentration. The first zone can include the portion of the underfill layer between the distal ends of the electrical couplers, and the second zone can include the portion of the underfill layer between the distal ends of the electrical couplers and a distal surface of the underfill layer. Alternatively, the underfill layer can include a first zone and a plurality of second zones. The second zones can be generally hemispherical and extend between the distal surface of the underfill layer and the distal ends of the electrical couplers.
Another aspect of the invention is directed to a method for disposing underfill material on a microelectronic device having a plurality of electrical couplers. In one embodiment, the method includes depositing an underfill layer onto the microelectronic device and covering at least a portion of the electrical couplers. The underfill layer includes a binder and a plurality of electrically charged filler elements in the binder. The method further includes applying an electric field to the underfill layer to manipulate at least a portion of the electrically charged filler elements. In one aspect of this embodiment, applying the electric field includes moving at least a portion of the electrically charged filler elements from a first zone into a second zone.
Another aspect of the invention is directed to a method of underfilling a microelectronic device assembly including a microelectronic component, a substrate, and a plurality of electrical couplers coupling the microelectronic component to the substrate. In one embodiment, the method includes disposing an underfill layer including a plurality of electrically charged filler elements between the microelectronic component and the substrate and moving at least a portion of the plurality of electrically charged filler elements within the underfill layer by applying an electric field to the underfill layer.